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xStack DES-3800 Series Layer 3 Stackable Fast Ethernet Managed Switch
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Hello Packet
Hello packets are OSPF packet type 1. They are sent periodically on all interfaces, including virtual links, in order to establish and
maintain neighbor relationships. In addition, Hello Packets are multicast on those physical networks having a multicast or
broadcast capability, enabling dynamic discovery of neighboring routers.
All routers connected to a common network must agree on certain parameters such as the Network Mask, the Hello Interval, and
the Router Dead Interval. These parameters are included in the hello packets, so that differences can inhibit the forming of
neighbor relationships. A detailed explanation of the receive process for Hello packets is necessary so that differences can inhibit
the forming of neighbor relationships.
The format of the Hello packet is shown below:
Hello Packet
1
Packet Length
Router ID
Area ID
Version No.
Checksum Authentication Type
Authentication
Authentication
Network Mask
Hello Interval
Options Router Priority
Router Dead Interval
Designated Router
Backup Designated Router
Neighbor
Figure 8- 20. Hello Packet
Field Description
Network Mask The network mask associated with this interface.
Options The optional capabilities supported by the router.
Hello Interval The number of seconds between this router’s Hello packets.
Router Priority This router’s Router Priority. The Router Priority is used in the
election of the DR and BDR. If this field is set to 0, the router is
ineligible become the DR or the BDR.
Router Dead Interval The number of seconds that must pass before declaring a
silent router as down.
Designated Router The identity of the DR for this network, in the view of the
advertising router. The DR is identified here by its IP interface
address on the network.
Backup Designated Router The identity of the Backup Designated Router (BDR) for this
network. The BDR is identified here by its IP interface address
on the network. This field is set to 0.0.0.0 if there is no BDR.
Neighbor The Router IDs of each router from whom valid Hello packets
have been seen within the Router Dead Interval on the
network.